The present disclosure relates generally to computer systems, and, more particularly, to a system and method for terminating the processor bus of multiprocessor computer systems.
A computer system generally includes various system components that are coupled together using one or more interconnected buses. As an example, a computer system may include a processor that is coupled to a processor bus. Also coupled to the processor bus is a memory controller bridge, which couples the processor bus to system memory, and a PCI bridge, that couples the processor bus to the PCI bus of the computer system. In the case of multi-processor computer systems, two or more processors may be coupled to the processor bus.
As processors, memories, and other system components increase in speed, the buses of the computer system begin to behave like transmission lines. As transmission frequencies and edge rates increase, signal integrity problems on the buses of the computer system are exacerbated. Reflections, false signal edges, incorrect voltage levels on the bus can cause false triggering, contribute to the transfer of erroneous data, and contribute to signal jitter. With respect to the processor bus, these signal integrity problems are often worse in multiprocessor systems because of the number of devices that are coupled to the bus.
In an attempt to solve the signal integrity problems, recent bus designs have placed strict controls on many of the electrical and mechanical parameters of the bus. In the case of high frequency bus communications, bus termination is necessary. Termination is often a requirement of bus standards to insure impedance matching across all of the loads of the bus. A processor bus standard that requires bus termination is the Gunning Transceiver Logic (GTL) standard and its successors, Gunning Transceiver Logic Plus (GTL+) and Assisted Gunning Transceiver Logic Plus (AGTL+). The communications frequency of a processor bus operating according to the AGTL+ standard can reach higher than 100 MHz. In this environment, if the processor bus is not terminated there may be an impedance mismatch between the processor bus itself and the processor or load. If there is an impedance mismatch some of the energy of the signal will be reflected back to the transmission line from the load. The effects of the reflection can be seen on the bus until the reflection is dissipated by the impedance of the loads of the processor bus or the processor bus itself.
The AGTL+ processor bus standard and its predecessors are designed to accommodate lower voltage swings. The AGTL+ bus standard includes open drain buffers at the ends of the bus that require pull-up resistors to terminate the bus. It is preferred in the AGTL+ bus standard that a processor reside on each end of the bus. Each processor package, which includes the CPU unit and may include other components, such as an L2 cache, includes the termination resistor for the bus. In this scenario in the case of a dual processor system, a processor package is installed in each of the two processor sockets on the printed circuit board. A difficulty arises, however, when the computer system is configured to include only a single processor. In this case, only the end of the bus that is coupled to the processor is terminated. Because the other end of the bus is coupled only to an empty processor socket, this end of the bus is not terminated.
A variety of techniques have been developed to provide for termination of the processor bus in single processor systems that include printed circuit boards that include two processor sockets. One technique is to couple a termination card to the empty processor socket. Termination cards are often costly and provide little functionality other than termination of the processor bus. The use of a termination card in an empty processor socket adds at least one more step to the manufacturing process, which necessarily introduces the possibility of error in the manufacture of a computer system. If a termination card is not installed in the empty processor socket in a single processor computer system, the computer system will likely not pass its system test and will have to be passed through an extra manufacturing or repair process. As such, the use of a termination card in an empty processor socket requires that the computer system manufacturer determine that a termination card must be installed and then correctly install the termination card.
As an alternative to the use of a termination card, some computer systems include a termination device that is coupled to the printed circuit board. An example of such a device is the GTL2020 terminator device manufactured by Philips Semiconductors of Sunnyvale, Calif. A terminator device is coupled to the end points of the processor bus and switches a termination resistor in and out of contact with the processor bus depending upon whether a processor is located in the second and potentially vacant processor socket. If a processor is installed in the second processor socket, the termination device switches a termination resistor out of contact with the processor bus, and the processor bus is terminated by a termination resistor in the package of the second processor. If a processor is not installed in the second processor socket, a termination resistor in the termination device is placed in contact with the processor bus, thereby terminating the processor bus. The use of a termination device, however, introduces a set of complications. Because a termination device is placed on the printed circuit board, the termination device will consume valuable space on an already crowded printed circuit board. Further, any distance between the second processor socket and the termination device will introduce capacitive effects on the transmission lines of the processor bus, thereby degrading the signal integrity of the processor bus. In many cases, it is difficult to place the termination device at a short distance from the second processor socket because of spacing requirements from the processor on the printed circuit board. Moreover, termination devices may be expensive and add another step to the process of manufacturing a computer system.
In accordance with the present disclosure, disclosed system and method for terminating the processor bus of a computer system provides significant advantages over current processor bus termination techniques. The processor bus of the present disclosure is terminated at one end by a primary processor and at a second end by an external termination resistor coupled between the processor bus and +V power at the connection of the processor bus and processor socket on the printed circuit board that receives a second processor in a dual processor system. Because the second processor includes an onboard termination resistor, the cumulative value of the termination resistance at the processor bus adjacent to the optional processor socket in a dual processor configuration is the parallel combination of the external termination resistor and the onboard termination resistor. In a single processor configuration, the value of the termination resistance at the processor bus adjacent to the optional processor socket is the value of the external termination resistor. The values of the external termination resistor and onboard termination resistor of the second processor are selected such that the end of the processor bus adjacent the optional processor socket is terminated irrespective of whether the computer system is configured as a single processor system or a dual processor system.
The bus processor termination technique disclosed herein is advantageous because it permits single processor computer systems to be manufactured with a dual processor printed circuit board without the installation of expensive termination cards or termination devices. Aside from being costly, termination devices consume valuable space on the printed circuit board of the computer system, and introduce capacitive effects on the processor bus. Termination cards are also costly and extra steps to the manufacturing process. Another advantage of the computer system is that the use of a single termination resistor at the location of the optional processor socket is an inexpensive means of terminating the processor bus. Other technical advantages will be apparent to those of ordinary skill in the art in view of the following specification, claims, and drawings.